1. Field of the Invention
The present invention relates to a radiation image storage panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor.
2. Description of the Prior Art
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and an intensifying screen. As a method replacing the conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In this method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film of a polymer material is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer generally comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described hereinbefore, and it is desired for the radiation image storage panel employed in the method to have a high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.), as well as the radiographic intensifying screen employed in the conventional radiography. Especially when the object is a human body, the sensitivity of the panel is desired to be increased, even if the level is low, for the purpose of reducing the radiation dose applied to the human body.
The sensitivity of the radiation image storage panel is essentially determined by the amount of stimulated emission given by the stimulable phosphor contained in the panel, and the emission amount varies depending upon not only the emission characteristics of the phosphor per se but also an intensity of stimulating rays for causing the phosphor to give stimulated emission when the intensity thereof is not sufficient.
In the radiation image recording and reproducing method, the radiation image storage panel is practically read out by scanning the surface of the panel with stimulating rays such as a laser beam. A portion of the stimulating rays is scattered in the panel, particularly in the phosphor layer, and released from the panel surface without exciting the stimulable phosphor, so that the phosphor is not sufficiently excited with the stimulating rays and the stimulating rays are not always employed efficiently in the method. Especially in the case of using a laser having a small power as a source of stimulating rays, it is desired to efficiently employ stimulating rays so as to enhance the sensitivity of the panel.
There has been filed a patent application for a radiation image storage panel on the surface of which an anti-reflecting film comprises an inorganic material or the like is provided (Japanese Patent Provisional Publication No. 61(1986)-164200, whose content is described in U.S. patent application No. 818,239 now U.S. Pat. No. 4,645,721 and in European Patent Application No. 86100417.4). The provision of the anti-reflecting film is intended to prevent the irradiated stimulating rays from being reflected by the panel surface and the anti-reflecting film is a thin film merely having a low reflectance for the stimulating rays.